Vitamin D is an extremely important vitamin that greatly affects various systems in your body ( 1 ).

Unlike other vitamins, vitamin D acts like a hormone and every single cell in your body has a receptor for it.

Your body makes it from cholesterol when your skin is exposed to sunlight.

It's also found in certain foods like fatty fish and dairy products, although getting enough from diet alone is very difficult.

The Recommended Daily Allowance (RDI) is usually between 400 and 800 IU, but many experts say you should get even more.

Vitamin D deficiency is very common. It is estimated that around 1 billion people worldwide have low levels of this vitamin in their blood ( 2 ).

According to a 2011 study, 41.6% of US adults are deficient. This number increases to 69.2% for Hispanics and 82.1% for African Americans ( 3 ).

Here are 7 common risk factors for vitamin D deficiency:

• Having dark skin
• Being older
• Being overweight or obese.
• Don't eat a lot of fish or dairy products.
• Far from the equator, where there is little sun all year round.
• Always use sunscreen when you go out.
• Stay inside.

People who live near the equator and have more sun exposure are less likely to be deficient because their skin produces enough vitamin D to meet their body's needs.

Most people are unaware that they are deficient as the symptoms are generally subtle. You may not recognize them easily, even if they significantly affect your quality of life.

Here are 8 signs and symptoms of vitamin D deficiency.

One of vitamin D's most important functions is maintaining a strong immune system, allowing you to fight off viruses and bacteria that cause disease.

It interacts directly with the cells responsible for fighting infection ( 4).

If you get sick frequently, especially with colds or the flu, low vitamin D levels can contribute.

Several large observational studies have shown an association between deficiency and respiratory infections such as common cold, bronchitis and pneumonia ( 5 , 6 ).

Several studies have shown that taking vitamin D supplements at doses of up to 4,000 IU daily can reduce the risk of respiratory infections ( 7 , 8 , 9 ).

In a study of people with the chronic lung disease COPD, only people who were severely vitamin D deficient had a significant benefit after taking a high-dose supplement for a year (10) .

Fatigue can have many causes, and vitamin D deficiency can be one of them.

Unfortunately, it is often overlooked as a possible cause.

Case studies have shown that very low blood levels can cause fatigue that severely impairs quality of life ( 11 , 12 ).

In one case, vitamin D blood levels were found to be as low as 5.9 ng/mL in a woman complaining of chronic daytime sleepiness and headaches. This is extremely low as anything below 20 ng/mL is considered deficient.

When the woman took a vitamin D supplement, her levels rose to 39 ng/ml and her symptoms disappeared ( 12 ).

However, even blood values ​​that are not extremely low can have a negative impact on your energy.

A large observational study investigated the association between vitamin D and fatigue in young women.

The study found that women with blood levels below 20 ng/mL or 21–29 ng/mL complained of fatigue more often than women with blood levels above 30 ng/mL (13).

Another observational study of female nurses found a strong association between low levels of vitamin D and fatigue.

Moreover, the researchers found that 89% of the nurses were deficient ( 14 ).

For more information on reducing fatigue, check out the 11 Best Vitamins and Supplements.

Vitamin D contributes to the maintenance of bone health in a number of ways.

For one, it improves calcium absorption in the body.

Bone pain and lower back pain can be signs of insufficient levels of vitamin D in the blood.

Large observational studies have shown an association between deficiency and chronic back pain ( 15 , 16 , 17 ).

One study looked at the link between vitamin D levels and back pain in over 9,000 older women.

Researchers found that people with deficiency were more likely to have back pain, including severe back pain, that restricted their daily activities ( 17 )

In a controlled study, people with vitamin D deficiency were almost twice as likely to have bone pain in their legs, ribs, or joints compared to those with normal blood counts ( 18 ).

A depressive mood can also be an indication of a vitamin D deficiency.

In review studies, researchers have linked vitamin D deficiency to depression, particularly in older adults ( 19 , 20 ).

In one analysis, 65% of observational studies found an association between low blood levels and depression.

On the other hand, most controlled studies, which have more scientific weight than observational studies, showed no association between the two studies ( 19 ).

However, the researchers who analyzed the studies found that the doses of vitamin D in controlled studies were often very low.

Moreover, they observed that some of the studies may not have lasted long enough to identify the effects of supplement intake on mood.

Some controlled studies have shown that vitamin D supplementation helps improve depression in people who are deficient, including seasonal depression that occurs in the colder months ( 21 , 22 ).

Slow wound healing after surgery or injury can be a sign that your vitamin D levels are too low.

Results from a test tube study suggest that the vitamin increases the production of compounds critical to the formation of new skin as part of the wound healing process ( 23 )

A study of people undergoing dental surgery found that certain aspects of healing were impaired by vitamin D deficiency ( 24 ).

It has also been suggested that vitamin D's role in fighting inflammation and fighting infection is important for proper healing.

An analysis examined patients with diabetic foot infections.

It has been found that individuals with severe vitamin D deficiency are more likely to have pro-inflammatory markers that may compromise healing ( 25 ).

Unfortunately, at this time, there is little research on the effects of vitamin D supplements on wound healing in people who are deficient.

However, one study found that in patients with vitamin D deficiency who were treated with the vitamin for leg ulcers, the ulcer size was reduced by an average of 28% ( 26 ).

Vitamin D plays a crucial role in calcium absorption and bone metabolism.

Many older people who are diagnosed with bone loss believe they need to take more calcium. However, they can also be deficient in vitamin D.

Low bone density is an indication that your bones have lost calcium and other minerals. This leads to an increased risk of fractures in older adults, especially women.

In a large observational study of over 1,100 middle-aged women going through menopause or postmenopause, researchers found a strong association between low vitamin D levels and low bone mineral density ( 27 ).

However, a controlled study found that women with vitamin D deficiency showed no improvement in bone mineral density when taking high-dose supplements, even when their blood levels improved (28).

Regardless of these findings, adequate vitamin D intake and maintaining blood levels in the optimal range may be a good strategy to protect bone mass and reduce fracture risk.

Hair loss is often attributed to stress, which is certainly a common cause.

However, severe hair loss may be due to disease or nutrient deficiencies.

Hair loss in women is associated with low vitamin D levels, although little research has been done on this ( 29 ).

Alopecia areata is an autoimmune disease characterized by severe hair loss on the head and other parts of the body It is associated with rickets, a condition that causes soft bones in children due to vitamin D deficiency ( 30 ) .

Low vitamin D levels are associated with alopecia areata and may be a risk factor for developing the disease ( 31 , 32 , 33 ).

A study of patients with alopecia areata showed that lower vitamin D blood levels tended to be associated with greater hair loss ( 33 ).

In a case study, topical application of a synthetic form of the vitamin was found to successfully treat hair loss in a young boy with a vitamin D receptor defect ( 34 ).

Many other foods and nutrients can affect the health of your hair. If you suffer from hair loss, you may be interested in 14 Best Foods For Hair Growth.

The causes of muscle pain are often difficult to pinpoint.

There is evidence that vitamin D deficiency may be a possible cause of muscle pain in children and adults ( 35 , 36 , 37 ).

In one study, 71% of people with chronic pain were found to be deficient ( 37 ).

The vitamin D receptor is found in nerve cells called nociceptors that sense pain.

A study in rats showed that deficiency caused pain and tenderness due to stimulation of nociceptors in the muscles ( 38 ).

Some studies have shown that taking high-dose vitamin D supplements can reduce various types of pain in people who are deficient ( 39 , 40 ).

A study of 120 vitamin D-deficient children who presented with growing pains found that a single vitamin dose reduced pain by an average of 57% ( 40 ).

Vitamin D deficiency is incredibly common and most people are unaware of it.

This is because the symptoms are often subtle and non-specific, meaning it's difficult to know if they're being caused by low vitamin D levels or something else.

If you think you may be deficient, it is important that you speak to your doctor and have your blood levels checked.

Fortunately, vitamin D deficiency is usually easy to fix.

You can either increase your sun exposure or eat more vitamin D-rich foods like fatty fish or fortified dairy products.You can also find a variety of vitamin D supplements on Amazon

Correcting your deficiency is simple, easy, and can have major health benefits.